Move associated type bounds check to ast lowering

This makes the check for when associated type bounds more accurate

compiler/rustc_ast_lowering/locales/en-US.ftl

@@ -19,6 +19,9 @@ ast_lowering_remove_parentheses = remove these parentheses
 ast_lowering_misplaced_impl_trait =
     `impl Trait` only allowed in function and inherent method return types, not in {$position}
 
+ast_lowering_misplaced_assoc_ty_binding =
+    associated type bounds are only allowed in where clauses and function signatures, not in {$position}
+
 ast_lowering_rustc_box_attribute_error =
     #[rustc_box] requires precisely one argument and no other attributes are allowed
 

compiler/rustc_ast_lowering/src/errors.rs

@@ -79,6 +79,14 @@ pub struct MisplacedImplTrait<'a> {
     pub position: DiagnosticArgFromDisplay<'a>,
 }
 
+#[derive(Diagnostic)]
+#[diag(ast_lowering_misplaced_assoc_ty_binding)]
+pub struct MisplacedAssocTyBinding<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub position: DiagnosticArgFromDisplay<'a>,
+}
+
 #[derive(Diagnostic, Clone, Copy)]
 #[diag(ast_lowering_rustc_box_attribute_error)]
 pub struct RustcBoxAttributeError {

compiler/rustc_ast_lowering/src/lib.rs

@@ -1002,8 +1002,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         } else {
             self.arena.alloc(hir::GenericArgs::none())
         };
-        let itctx_tait = &ImplTraitContext::TypeAliasesOpaqueTy;
-
         let kind = match &constraint.kind {
             AssocConstraintKind::Equality { term } => {
                 let term = match term {
@@ -1040,7 +1038,13 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     // then to an opaque type).
                     //
                     // FIXME: this is only needed until `impl Trait` is allowed in type aliases.
-                    ImplTraitContext::Disallowed(_) if self.is_in_dyn_type => (true, itctx_tait),
+                    ImplTraitContext::Disallowed(position) if self.is_in_dyn_type => {
+                        self.tcx.sess.emit_err(errors::MisplacedAssocTyBinding {
+                            span: constraint.span,
+                            position: DiagnosticArgFromDisplay(position),
+                        });
+                        (false, itctx)
+                    }
 
                     // We are in the parameter position, but not within a dyn type:
                     //

compiler/rustc_ast_passes/locales/en-US.ftl

@@ -11,9 +11,6 @@ ast_passes_forbidden_let_stable =
 ast_passes_deprecated_where_clause_location =
     where clause not allowed here
 
-ast_passes_forbidden_assoc_constraint =
-    associated type bounds are not allowed within structs, enums, or unions
-
 ast_passes_keyword_lifetime =
     lifetimes cannot use keyword names
 

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -71,10 +71,6 @@ struct AstValidator<'a> {
     /// or `Foo::Bar<impl Trait>`
     is_impl_trait_banned: bool,
 
-    /// Used to ban associated type bounds (i.e., `Type<AssocType: Bounds>`) in
-    /// certain positions.
-    is_assoc_ty_bound_banned: bool,
-
     /// See [ForbiddenLetReason]
     forbidden_let_reason: Option<ForbiddenLetReason>,
 
@@ -180,30 +176,12 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    fn with_banned_assoc_ty_bound(&mut self, f: impl FnOnce(&mut Self)) {
-        let old = mem::replace(&mut self.is_assoc_ty_bound_banned, true);
-        f(self);
-        self.is_assoc_ty_bound_banned = old;
-    }
-
     fn with_impl_trait(&mut self, outer: Option<Span>, f: impl FnOnce(&mut Self)) {
         let old = mem::replace(&mut self.outer_impl_trait, outer);
         f(self);
         self.outer_impl_trait = old;
     }
 
-    fn visit_assoc_constraint_from_generic_args(&mut self, constraint: &'a AssocConstraint) {
-        match constraint.kind {
-            AssocConstraintKind::Equality { .. } => {}
-            AssocConstraintKind::Bound { .. } => {
-                if self.is_assoc_ty_bound_banned {
-                    self.session.emit_err(ForbiddenAssocConstraint { span: constraint.span });
-                }
-            }
-        }
-        self.visit_assoc_constraint(constraint);
-    }
-
     // Mirrors `visit::walk_ty`, but tracks relevant state.
     fn walk_ty(&mut self, t: &'a Ty) {
         match &t.kind {
@@ -1248,7 +1226,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                         // are allowed to contain nested `impl Trait`.
                         AngleBracketedArg::Constraint(constraint) => {
                             self.with_impl_trait(None, |this| {
-                                this.visit_assoc_constraint_from_generic_args(constraint);
+                                this.visit_assoc_constraint(constraint);
                             });
                         }
                     }
@@ -1373,14 +1351,6 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
         visit::walk_param_bound(self, bound)
     }
 
-    fn visit_variant_data(&mut self, s: &'a VariantData) {
-        self.with_banned_assoc_ty_bound(|this| visit::walk_struct_def(this, s))
-    }
-
-    fn visit_enum_def(&mut self, enum_definition: &'a EnumDef) {
-        self.with_banned_assoc_ty_bound(|this| visit::walk_enum_def(this, enum_definition))
-    }
-
     fn visit_fn(&mut self, fk: FnKind<'a>, span: Span, id: NodeId) {
         // Only associated `fn`s can have `self` parameters.
         let self_semantic = match fk.ctxt() {
@@ -1709,7 +1679,6 @@ pub fn check_crate(session: &Session, krate: &Crate, lints: &mut LintBuffer) ->
         outer_impl_trait: None,
         disallow_tilde_const: None,
         is_impl_trait_banned: false,
-        is_assoc_ty_bound_banned: false,
         forbidden_let_reason: Some(ForbiddenLetReason::GenericForbidden),
         lint_buffer: lints,
     };

compiler/rustc_ast_passes/src/errors.rs

@@ -23,13 +23,6 @@ pub struct ForbiddenLetStable {
     pub span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(ast_passes_forbidden_assoc_constraint)]
-pub struct ForbiddenAssocConstraint {
-    #[primary_span]
-    pub span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(ast_passes_keyword_lifetime)]
 pub struct KeywordLifetime {

tests/ui/associated-type-bounds/bad-universal-in-dyn-in-where-clause.rs

@@ -0,0 +1,15 @@
+#![feature(associated_type_bounds)]
+
+trait B {
+    type AssocType;
+}
+
+fn f()
+where
+    dyn for<'j> B<AssocType: 'j>:,
+    //~^ ERROR associated type bounds are only allowed in where clauses and function signatures
+    //~| ERROR the value of the associated type `AssocType` (from trait `B`) must be specified
+{
+}
+
+fn main() {}

tests/ui/associated-type-bounds/bad-universal-in-dyn-in-where-clause.stderr

@@ -0,0 +1,18 @@
+error: associated type bounds are only allowed in where clauses and function signatures, not in bound
+  --> $DIR/bad-universal-in-dyn-in-where-clause.rs:9:19
+   |
+LL |     dyn for<'j> B<AssocType: 'j>:,
+   |                   ^^^^^^^^^^^^^
+
+error[E0191]: the value of the associated type `AssocType` (from trait `B`) must be specified
+  --> $DIR/bad-universal-in-dyn-in-where-clause.rs:9:9
+   |
+LL |     type AssocType;
+   |     -------------- `AssocType` defined here
+...
+LL |     dyn for<'j> B<AssocType: 'j>:,
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^ help: specify the associated type: `for<'j> B<AssocType: 'j, AssocType = Type>`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0191`.

tests/ui/associated-type-bounds/bad-universal-in-impl-sig.rs

@@ -0,0 +1,14 @@
+#![feature(associated_type_bounds)]
+
+trait Trait {
+    type Item;
+}
+
+trait Trait2 {}
+
+// It's not possible to insert a universal `impl Trait` here!
+impl dyn Trait<Item: Trait2> {}
+//~^ ERROR associated type bounds are only allowed in where clauses and function signatures
+//~| ERROR the value of the associated type `Item` (from trait `Trait`) must be specified
+
+fn main() {}

tests/ui/associated-type-bounds/bad-universal-in-impl-sig.stderr

@@ -0,0 +1,8 @@
+error: associated type bounds are only allowed in where clauses and function signatures, not in impl header
+  --> $DIR/bad-universal-in-impl-sig.rs:10:16
+   |
+LL | impl dyn Trait<Item: Trait2> {}
+   |                ^^^^^^^^^^^^
+
+error: aborting due to previous error
+

tests/ui/associated-type-bounds/inside-adt.rs

@@ -3,28 +3,33 @@
 use std::mem::ManuallyDrop;
 
 struct S1 { f: dyn Iterator<Item: Copy> }
-//~^ ERROR associated type bounds are not allowed within structs, enums, or unions
+//~^ ERROR associated type bounds are only allowed in where clauses and function signatures
+//~| ERROR the value of the associated type `Item` (from trait `Iterator`) must be specified
 struct S2 { f: Box<dyn Iterator<Item: Copy>> }
-//~^ ERROR associated type bounds are not allowed within structs, enums, or unions
+//~^ ERROR associated type bounds are only allowed in where clauses and function signatures
+//~| ERROR the value of the associated type `Item` (from trait `Iterator`) must be specified
 struct S3 { f: dyn Iterator<Item: 'static> }
-//~^ ERROR associated type bounds are not allowed within structs, enums, or unions
+//~^ ERROR associated type bounds are only allowed in where clauses and function signatures
+//~| ERROR the value of the associated type `Item` (from trait `Iterator`) must be specified
 
 enum E1 { V(dyn Iterator<Item: Copy>) }
-//~^ ERROR associated type bounds are not allowed within structs, enums, or unions
+//~^ ERROR associated type bounds are only allowed in where clauses and function signatures
-//~| ERROR the size for values of type `(dyn Iterator<Item = impl Copy> + 'static)`
+//~| ERROR the value of the associated type `Item` (from trait `Iterator`) must be specified
 enum E2 { V(Box<dyn Iterator<Item: Copy>>) }
-//~^ ERROR associated type bounds are not allowed within structs, enums, or unions
+//~^ ERROR associated type bounds are only allowed in where clauses and function signatures
+//~| ERROR the value of the associated type `Item` (from trait `Iterator`) must be specified
 enum E3 { V(dyn Iterator<Item: 'static>) }
-//~^ ERROR associated type bounds are not allowed within structs, enums, or unions
+//~^ ERROR associated type bounds are only allowed in where clauses and function signatures
-//~| ERROR the size for values of type `(dyn Iterator<Item = impl Sized + 'static> + 'static)`
+//~| ERROR the value of the associated type `Item` (from trait `Iterator`) must be specified
 
 union U1 { f: ManuallyDrop<dyn Iterator<Item: Copy>> }
-//~^ ERROR associated type bounds are not allowed within structs, enums, or unions
+//~^ ERROR associated type bounds are only allowed in where clauses and function signatures
-//~| ERROR the size for values of type `(dyn Iterator<Item = impl Copy> + 'static)`
+//~| ERROR the value of the associated type `Item` (from trait `Iterator`) must be specified
 union U2 { f: ManuallyDrop<Box<dyn Iterator<Item: Copy>>> }
-//~^ ERROR associated type bounds are not allowed within structs, enums, or unions
+//~^ ERROR associated type bounds are only allowed in where clauses and function signatures
+//~| ERROR the value of the associated type `Item` (from trait `Iterator`) must be specified
 union U3 { f: ManuallyDrop<dyn Iterator<Item: 'static>> }
-//~^ ERROR associated type bounds are not allowed within structs, enums, or unions
+//~^ ERROR associated type bounds are only allowed in where clauses and function signatures
-//~| ERROR the size for values of type `(dyn Iterator<Item = impl Sized + 'static> + 'static)`
+//~| ERROR the value of the associated type `Item` (from trait `Iterator`) must be specified
 
 fn main() {}

tests/ui/associated-type-bounds/inside-adt.stderr

@@ -1,131 +1,129 @@
-error: associated type bounds are not allowed within structs, enums, or unions
+error: associated type bounds are only allowed in where clauses and function signatures, not in field type
   --> $DIR/inside-adt.rs:5:29
    |
 LL | struct S1 { f: dyn Iterator<Item: Copy> }
    |                             ^^^^^^^^^^
 
-error: associated type bounds are not allowed within structs, enums, or unions
+error: associated type bounds are only allowed in where clauses and function signatures, not in field type
-  --> $DIR/inside-adt.rs:7:33
+  --> $DIR/inside-adt.rs:8:33
    |
 LL | struct S2 { f: Box<dyn Iterator<Item: Copy>> }
    |                                 ^^^^^^^^^^
 
-error: associated type bounds are not allowed within structs, enums, or unions
+error: associated type bounds are only allowed in where clauses and function signatures, not in field type
-  --> $DIR/inside-adt.rs:9:29
+  --> $DIR/inside-adt.rs:11:29
    |
 LL | struct S3 { f: dyn Iterator<Item: 'static> }
    |                             ^^^^^^^^^^^^^
 
-error: associated type bounds are not allowed within structs, enums, or unions
+error: associated type bounds are only allowed in where clauses and function signatures, not in field type
-  --> $DIR/inside-adt.rs:12:26
+  --> $DIR/inside-adt.rs:15:26
    |
 LL | enum E1 { V(dyn Iterator<Item: Copy>) }
    |                          ^^^^^^^^^^
 
-error: associated type bounds are not allowed within structs, enums, or unions
+error: associated type bounds are only allowed in where clauses and function signatures, not in field type
-  --> $DIR/inside-adt.rs:15:30
+  --> $DIR/inside-adt.rs:18:30
    |
 LL | enum E2 { V(Box<dyn Iterator<Item: Copy>>) }
    |                              ^^^^^^^^^^
 
-error: associated type bounds are not allowed within structs, enums, or unions
+error: associated type bounds are only allowed in where clauses and function signatures, not in field type
-  --> $DIR/inside-adt.rs:17:26
+  --> $DIR/inside-adt.rs:21:26
    |
 LL | enum E3 { V(dyn Iterator<Item: 'static>) }
    |                          ^^^^^^^^^^^^^
 
-error: associated type bounds are not allowed within structs, enums, or unions
+error: associated type bounds are only allowed in where clauses and function signatures, not in field type
-  --> $DIR/inside-adt.rs:21:41
+  --> $DIR/inside-adt.rs:25:41
    |
 LL | union U1 { f: ManuallyDrop<dyn Iterator<Item: Copy>> }
    |                                         ^^^^^^^^^^
 
-error: associated type bounds are not allowed within structs, enums, or unions
+error: associated type bounds are only allowed in where clauses and function signatures, not in field type
-  --> $DIR/inside-adt.rs:24:45
+  --> $DIR/inside-adt.rs:28:45
    |
 LL | union U2 { f: ManuallyDrop<Box<dyn Iterator<Item: Copy>>> }
    |                                             ^^^^^^^^^^
 
-error: associated type bounds are not allowed within structs, enums, or unions
+error: associated type bounds are only allowed in where clauses and function signatures, not in field type
-  --> $DIR/inside-adt.rs:26:41
+  --> $DIR/inside-adt.rs:31:41
    |
 LL | union U3 { f: ManuallyDrop<dyn Iterator<Item: 'static>> }
    |                                         ^^^^^^^^^^^^^
 
-error[E0277]: the size for values of type `(dyn Iterator<Item = impl Copy> + 'static)` cannot be known at compilation time
+error[E0191]: the value of the associated type `Item` (from trait `Iterator`) must be specified
-  --> $DIR/inside-adt.rs:12:13
+  --> $DIR/inside-adt.rs:5:20
+   |
+LL | struct S1 { f: dyn Iterator<Item: Copy> }
+   |                    ^^^^^^^^^^^^^^^^^^^^
+   |                    |
+   |                    associated type `Item` must be specified
+   |                    help: specify the associated types: `Iterator<Item: Copy, Item = Type>`
+
+error[E0191]: the value of the associated type `Item` (from trait `Iterator`) must be specified
+  --> $DIR/inside-adt.rs:8:24
+   |
+LL | struct S2 { f: Box<dyn Iterator<Item: Copy>> }
+   |                        ^^^^^^^^^^^^^^^^^^^^
+   |                        |
+   |                        associated type `Item` must be specified
+   |                        help: specify the associated types: `Iterator<Item: Copy, Item = Type>`
+
+error[E0191]: the value of the associated type `Item` (from trait `Iterator`) must be specified
+  --> $DIR/inside-adt.rs:11:20
+   |
+LL | struct S3 { f: dyn Iterator<Item: 'static> }
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^ help: specify the associated type: `Iterator<Item: 'static, Item = Type>`
+
+error[E0191]: the value of the associated type `Item` (from trait `Iterator`) must be specified
+  --> $DIR/inside-adt.rs:15:17
    |
 LL | enum E1 { V(dyn Iterator<Item: Copy>) }
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |                 ^^^^^^^^^^^^^^^^^^^^
-   |
+   |                 |
-   = help: the trait `Sized` is not implemented for `(dyn Iterator<Item = impl Copy> + 'static)`
+   |                 associated type `Item` must be specified
-   = note: no field of an enum variant may have a dynamically sized type
+   |                 help: specify the associated types: `Iterator<Item: Copy, Item = Type>`
-   = help: change the field's type to have a statically known size
-help: borrowed types always have a statically known size
-   |
-LL | enum E1 { V(&dyn Iterator<Item: Copy>) }
-   |             +
-help: the `Box` type always has a statically known size and allocates its contents in the heap
-   |
-LL | enum E1 { V(Box<dyn Iterator<Item: Copy>>) }
-   |             ++++                        +
 
-error[E0277]: the size for values of type `(dyn Iterator<Item = impl Sized + 'static> + 'static)` cannot be known at compilation time
+error[E0191]: the value of the associated type `Item` (from trait `Iterator`) must be specified
-  --> $DIR/inside-adt.rs:17:13
+  --> $DIR/inside-adt.rs:18:21
+   |
+LL | enum E2 { V(Box<dyn Iterator<Item: Copy>>) }
+   |                     ^^^^^^^^^^^^^^^^^^^^
+   |                     |
+   |                     associated type `Item` must be specified
+   |                     help: specify the associated types: `Iterator<Item: Copy, Item = Type>`
+
+error[E0191]: the value of the associated type `Item` (from trait `Iterator`) must be specified
+  --> $DIR/inside-adt.rs:21:17
    |
 LL | enum E3 { V(dyn Iterator<Item: 'static>) }
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^ help: specify the associated type: `Iterator<Item: 'static, Item = Type>`
-   |
-   = help: the trait `Sized` is not implemented for `(dyn Iterator<Item = impl Sized + 'static> + 'static)`
-   = note: no field of an enum variant may have a dynamically sized type
-   = help: change the field's type to have a statically known size
-help: borrowed types always have a statically known size
-   |
-LL | enum E3 { V(&dyn Iterator<Item: 'static>) }
-   |             +
-help: the `Box` type always has a statically known size and allocates its contents in the heap
-   |
-LL | enum E3 { V(Box<dyn Iterator<Item: 'static>>) }
-   |             ++++                           +
 
-error[E0277]: the size for values of type `(dyn Iterator<Item = impl Copy> + 'static)` cannot be known at compilation time
+error[E0191]: the value of the associated type `Item` (from trait `Iterator`) must be specified
-  --> $DIR/inside-adt.rs:21:15
+  --> $DIR/inside-adt.rs:25:32
    |
 LL | union U1 { f: ManuallyDrop<dyn Iterator<Item: Copy>> }
-   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |                                ^^^^^^^^^^^^^^^^^^^^
-   |
+   |                                |
-   = help: within `ManuallyDrop<(dyn Iterator<Item = impl Copy> + 'static)>`, the trait `Sized` is not implemented for `(dyn Iterator<Item = impl Copy> + 'static)`
+   |                                associated type `Item` must be specified
-   = note: required because it appears within the type `ManuallyDrop<dyn Iterator<Item = impl Copy>>`
+   |                                help: specify the associated types: `Iterator<Item: Copy, Item = Type>`
-   = note: no field of a union may have a dynamically sized type
-   = help: change the field's type to have a statically known size
-help: borrowed types always have a statically known size
-   |
-LL | union U1 { f: &ManuallyDrop<dyn Iterator<Item: Copy>> }
-   |               +
-help: the `Box` type always has a statically known size and allocates its contents in the heap
-   |
-LL | union U1 { f: Box<ManuallyDrop<dyn Iterator<Item: Copy>>> }
-   |               ++++                                      +
 
-error[E0277]: the size for values of type `(dyn Iterator<Item = impl Sized + 'static> + 'static)` cannot be known at compilation time
+error[E0191]: the value of the associated type `Item` (from trait `Iterator`) must be specified
-  --> $DIR/inside-adt.rs:26:15
+  --> $DIR/inside-adt.rs:28:36
+   |
+LL | union U2 { f: ManuallyDrop<Box<dyn Iterator<Item: Copy>>> }
+   |                                    ^^^^^^^^^^^^^^^^^^^^
+   |                                    |
+   |                                    associated type `Item` must be specified
+   |                                    help: specify the associated types: `Iterator<Item: Copy, Item = Type>`
+
+error[E0191]: the value of the associated type `Item` (from trait `Iterator`) must be specified
+  --> $DIR/inside-adt.rs:31:32
    |
 LL | union U3 { f: ManuallyDrop<dyn Iterator<Item: 'static>> }
-   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^ help: specify the associated type: `Iterator<Item: 'static, Item = Type>`
-   |
-   = help: within `ManuallyDrop<(dyn Iterator<Item = impl Sized + 'static> + 'static)>`, the trait `Sized` is not implemented for `(dyn Iterator<Item = impl Sized + 'static> + 'static)`
-   = note: required because it appears within the type `ManuallyDrop<dyn Iterator<Item = impl Sized>>`
-   = note: no field of a union may have a dynamically sized type
-   = help: change the field's type to have a statically known size
-help: borrowed types always have a statically known size
-   |
-LL | union U3 { f: &ManuallyDrop<dyn Iterator<Item: 'static>> }
-   |               +
-help: the `Box` type always has a statically known size and allocates its contents in the heap
-   |
-LL | union U3 { f: Box<ManuallyDrop<dyn Iterator<Item: 'static>>> }
-   |               ++++                                         +
 
-error: aborting due to 13 previous errors
+error: aborting due to 18 previous errors
 
-For more information about this error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0191`.